The incidence of Pneumocystis carinii cysts in the lung tissue of patients with AIDS, but no respiratory symptoms is being studied by evaluation of these patients with (1) history and physical examination, (2) chest X-ray, (3) Gallium scan, (4) pulmonary function testing, and (5) bronchoscopy. To date, five patients have been entered into this protocol. None have had pneumocystis carinii organisms identified, although all have had interstitital pneumonitis. The importance of this study is the need to understand the pathogenesis of pneumocystis pneumonia and to identify those patient populations that would most benefit from preventive therapy.